hi, recently i encounter a coating problem and don't know why it happened.

The base material is Aluminum 7075. The part was sent to machined and then underwent 'yellow chrome' (be informed by customer, he wanted yellow chrome as to prevent aluminum be oxidized. He said after yellow chrome, the part did not undergo any post treatment or heat treatment) and then sent to us for TBC. We used Amdry 962 as bond coat and Metco 204NS-G as top coat. After finish thermal spray, we wrapped it by stretch film (pallet film).

Customer said that after 3 or more months, the zirconia coating has many blistering and 'stain' on it, some of them have not yet unwrap.

What I know is after electroplating, the part must undergo heat treatment so as to release the chemicals or hydrogen inside. I suspect this is the reason cause it blistering.

Difficult one to answer with the available information. You will need to go through the history of the processes and later storage with a fine tooth comb to see if that shows any clues.
My gut feeling with this is that the chromating process has probably nothing to do with the failure. I think it is probably straight forward interface corrosion caused by poor storage conditions.

the ceramic coating should be inert to most of the environment. Chemicals will react with ceramic coatings (because after TBC coating, the customer sent the work-piece to another vendor for masking, cause need do painting on the coating)? If the zirconia coating is exposed to air-con environment, humidity high, the ceramic will have chemical reaction?

The ceramic coating itself should not suffer corrosion, but these coatings are porous and will give little barrier effect. Even the high temperature oxidation resistant bond coat may not give sufficient barrier to moisture penetration and the onset of corrosion of the substrate. In normal service this should not be a problem, but prolonged storage in poor conditions could be a problem. I have seen this effect with other ceramic coatings. The plastic wrapping used to protect the components may well be making conditions worse, by not allowing moisture to escape freely. You could question whether your other vendors could have contaminated the coating with chemicals that could corrode aluminium or nickel based bond coat. As you say the ceramic coating is relatively inert and should not corrode.

Due to the large batch of work-pieces, normally we sprayed all work-pieces with bond coat first, few days later then sprayed top coat (I know that it is not good practise, but it is very troublesome to keep-on changing powders, as we used old powder system, not like twin-powder system). Most of the work-pieces were sprayed by my colleagues. After sprayed bond coat, they did not wrap the work-piece, just put them in box ( I stay in Singapore, which the humidity is around 80% ). I'm not sure whether they have pre-heated before apply top coat or not.

The customer said they might use bare hand to touch the coatings when doing masking. No chemicals contact with the coatings.

Long time ago, we have sprayed some coupons for showing customers. However, the base materials are stainless steel. Somemore, some of the coupons were grinded. Hence, the coolants was inevitable went inside coating. However, so far that is no problem on the coatings. From your reply, I think that moisture is also one of the possibilities that causing blistering. Moisture might penetrate through the coating and react with the base material, Aluminum 7075, created chemical reactions, caused blistering.

You said you have seen this effect with other ceramic coatings, their base materials are aluminum too?

Its just like being a detective and I see you are starting to find clues to the possible cause of your blistered coatings.

Quote:You said you have seen this effect with other ceramic coatings, their base materials are aluminum too?

I have seen it with carbon steels, stainless steel and aluminium substrates. I'm not saying that it is a common occurence, but it does unfortuetly happen I suppose if the conditiond are right. One case I remember, similar to yours in that the components were transported by sea and then stored for 6 months. The dense chromium oxide coatings on steel rolls were blistered. After significant coating failure evaluation by metallogaphy, SEM and chemical analysis, also analysis of the packing materials, which only yielded that the corrosion products found under the blisters contained no foreign chemical species like chlorides or sulphides and that everything else was normal. At a later meeting to discuss all of the findings, I asked some questions about their process, particularly the so called vacuum sealing process for the coating. The sealing process turned out to be tubes filled with phenolic resin in which they submerged the component and then sealed and pulled a vacuum. Not really the ideal way to do it. Anyway I asked did they sealed the coating after spraying or after finish grinding, at this point the coating shop manager rush out of the meeting. The penny had dropped, they were sealing the coatings directly after grinding, sealing in moisture from the water based grinding coolant.

Ya. Even my most experience colleagues (can get 10-year awards already) did not see this blistering effect before. From your case, chromium oxide coatings on steel rolls were blistered too. Ceramic coatings are porous, chromium oxide coatings has "corrosion resistant" property. After stored for 6 months in moisture conditions, it blistered. How come the ceramic coatings so 'lousy' performance? Its application is for resist corrosion, now have not start 'doing the job', failed already. If put it in corrosive conditions, it should fail in lesser months only, then ceramic coating is useless.

Amdry 962 is used as protective coatings in hot corrosive or oxidizing environments at high temperature. That means it is insoluble in all acids & alkalis conditions? From your experience, in which condition (chemical reaction) it will fail?

Ceramic coatings, though very corrosion resistant in themselves, do not always provide complete barrier protection alone due to porosity. Metallic bond coats can help, but again you are relying on a barrier effect and this is not always 100%. In your case your Amdry 962 bond coat was probably not corroded, but failed as a barrier to corrosion (aqueous not thermal oxidation) of your substrate.

Quote:How come the ceramic coatings so 'lousy' performance? Its application is for resist corrosion, now have not start 'doing the job', failed already. If put it in corrosive conditions, it should fail in lesser months only, then ceramic coating is useless.

Unfortunately, this is where thermal spray coatings get a "bad name". They are not universally good for everything. They need to be carefully tailored to suit the service conditions and environment. Thermal barrier coatings like yours are designed to have a porous structure, so you can't expect them to act as a barrier. Amdry 962 (NiCrAlY) is designed to give good high temperature oxidation resistance, in fact at temperatures somewhat higher than the melting point of your aluminium substrate! so we could question whether Amdry 962 was the the best choice of bond coat for this particular application. Ceramic coatings (and most TS coatings for that matter, except sacrificial ones) normally need to be sealer treated to provide a good corrosion resistance barrier. So a well constructed ceramic coating system can and does provide good service in corrosive conditions while providing some of its other unique properties. So its not all doom and gloom

Yes, to some extent. An MCrAlY + TBC would normally be quite corrosion resistant and particularly resistant to high temperature oxidation, but even the best systems can not always resist conditions that we might first think of as being non-corrosive, such as in storage conditions where moisture or corrosive elements are enclose or sealed in by packing material. Just think of how you would feel if you were wrap in plastic cling film for 3 months, even without any initial corrosive elements being present. The trouble is, how do we define what is generically corrosive? We can't really, take a plain carbon steel, general considered poor as resisting corrosion (in contact with air and water), but it is good generally in alkali and strangely in highly oxidative conditions like concentrated nitric acid. Aluminium and stainless steels on the other hand are considered to have fairly good corrosion resistance (in contact with air and water), but not so good in alkali or reducing environments. The fact is that we can't really say that any particular material will resist corrosion in every environment. I suppose this is why these type of subjects becomes so interesting, as the questions and answers tend toward infinity. To some extent, I'm trying to say that you are just unlucky in your circumstances, rather than just saying the coating system rubbish.

Note: I've probably had a few too many tonight, so I may edit this later.

Except TBC, and coatings that need porosity to hold lubricant, can I say that the rest coatings is better to be sealed by coolant? Normally after HVOF spray (e.g. WC) on shaft journals for wear resistant, do you apply sealant before grinding (with coolant)? Is there any grinding machine that does not need coolant?

If for insulation purpose, any difference using sealant (MetcoSeal AP??) and without using sealant after plasma spray alumina?

Except TBC, and coatings that need porosity to hold lubricant, can I say that the rest coatings is better to be sealed by coolant? Normally after HVOF spray (e.g. WC) on shaft journals for wear resistant, do you apply sealant before grinding (with coolant)? Is there any grinding machine that does not need coolant?

If for insulation purpose, any difference using sealant (MetcoSeal AP??) and without using sealant after plasma spray alumina?

Generally, yes to your first question (assuming you mean sealant rather than coolant). It is a good idea to seal before wet machining/grinding to stop ingress of cutting/coolant fluids and abrasives into the coating. Then reapply sealer after finishing to make sure coating is properly sealed. Some coatings are machined/ground dry, but even here you could argue that it is best to pre-seal, unless of course you require open porosity. It could be argued that the denser HVOF coatings don't need to be sealed or that sealing these coating is ineffective (no or little penetration). Still many seal these coatings just to make sure ("belt and braces"). Sealing of ceramic coatings improves dielectric strength/electrical insulation in ambient conditions. Thermal insulation properties would probably be very slightly worse in dry ambient conditions, at much higher temperatures the sealants would burn out/decompose.